In drilling an oil well, the ordinary procedure involves positioning a drill stem including a string of pipe and drill bit in a borehole which is formed by the drill bit. A lubricant known as drilling mud (comprising primarily various clays and water) is circulated downwardly through the drill stem and is returned to the surface in the annular space on the exterior of the drill stem. Depending on the geology, certain formations may be encountered by the borehole which liberate hydrogen sulfide. The hydrogen sulfide is picked up in the drilling mud and carried to the surface. There are two particularly distinct problems arising from this. First of all, when the hydrogen sulfide enters the flowing drilling mud, it reacts with the ferrous materials comprising the drill stem. Hydrogen sulfide corrosively attacks most ferrous drill pipe. This exposes at least 95% of the drill pipe to damage from hydrogen sulfide. The corrosion damage is cumulative, and accelerates with an increase in concentration of hydrogen sulfide or temperature increase. It is therefore undesirable to expose the metal components in the borehole to hydrogen sulfide.
Even more importantly, the drilling mud is returned to the surface where it is pumped from the annular space into mud pits. In the mud pits, dissolved gas bubbles in the mud float to the surface and poison the atmosphere. This may expose personnel near the drilling rig to poisoning from the hydrogen sulfide gas. Hydrogen sulfide gas can be fatal to nearby personnel even in trace quantities. Moreover, if it escapes into the atmosphere, it may well attack vegetation also. Hydrogen sulfide is quite active chemically; it is undesirable to release any hydrogen sulfide to the atmosphere. For these reasons, it is very desirable to remove hydrogen sulfide to avoid damage to the equipment, possible death and destruction.
Inevitably, all entrained gas bubbles and dissolved gases in the drilling mud must be removed to control the weight of the drilling mud. When the mud becomes gas cut, it becomes light and frothy and is no longer able to provide the mud weight required for drilling procedures. In particular, this weight requirement requires constant monitoring to assure that the drilling mud that is recirculated from the annular space into the mud pits and back through the drill stem is not reduced in weight by gases absorbed during drilling.
Various efforts have been made in the past to deal with this problem. As an example, one such effort involves U.S. Pat. No. 4,011,304. This reference involves a gas treatment procedure not concerned with drilling mud. U.S. Pat. No. 4,220,585 is more aptly concerned with a viscosity control agent and sets forth titanium or zirconium lignosulfonates. In U.S. Pat. No. 4,278,646, an acid pH system is set forth. In U.S. Pat. No. 4,332,687, a heavy metal removal process is set forth. U.S. Pat. No. 4,252,655 is a zinc chelate system. As will be understood from these references, a downhole hydrogen sulfide scavenging system utilizing iron chelates in the drilling fluid (as set forth in greater detail hereinafter) is able to remove hydrogen sulfide from the drilling mud and accomplishes this in an improved and unobvious manner.
A variety of situations may be encountered in a drilling process. The mud may simply be circulated down through the drill stem and returned through the annular space. If that is the case, it is important to add to the mud processing equipment an aeration process step in which the mud is sprayed through the air to obtain chelate regeneration. Without regeneration, the iron chelate material will have to be added continuously. Recirculation of the chelate reduces the required supply of the chelate. This extends the life of the chelate and thereby reduces the needed quantity of additive. The present procedure sets forth an important step of regeneration of the iron chelate by exposing it to air (including oxygen) through the use of an aeration process step.
Sometimes during the drilling process, the mud may pick up particles which have to be removed with a device known as a shale shaker. The discharged stream of mud recovered from the well is transferred through suitable pipes to a shale shaker. The mud is poured over a conveyer belt. The conveyer belt is more aptly woven screen which permits the drilling mud to trickle through the perforations in the screen. The screen carries the particulate matter away for disposal. After the drilling mud picks up particles, they are removed by this procedure and while the mud flow is at the surface, the iron chelate mixed in the drilling mud is exposed to oxygen in the air for chelate regeneration.
In other instances of drilling a well, a formation may be encountered which produces a gas which is dissolved in the drilling mud. The gas may be completely free of hydrogen sulfide or may be mixed with it. When a well produces combustible gas mixed with hydrogen sulfide, the gas is called sour gas. The present invention contemplates the use of a degaser connected to the mud system. The degaser liberates gas bubbles from the mud. In the instance that sour gas is encountered, the iron chelate of this disclosure converts the gaseous hydrogen sulfide, this conversion typically occurring while the mud is flowing in the annular space back to the surface. The mud thereafter is introduced to the degaser whereupon bubbles of natural gas carried in the drilling mud are removed. Dissolved gas and bubbles are removed by permitting vaporization to atmosphere. While the surface degassing procedure removes combustible gases in the drilling mud, oxygen (typically by introduction of air) is permitted to bubble through the drilling mud and thereby regenerates the chelate.
Without regard to specifics, the present procedure contemplates the addition of an iron chelate to a flowing mud stream. It can be introduced as a solution in water or as a particulate ground to a suitable size and mixed in the drilling mud. So long as no hydrogen sulfide is encountered, the iron chelate can be recirculated indefinitely in the mud system with nominal circulation and chemical loss. When hydrogen sulfide is encountered, a conversion occurs as will be described whereupon hydrogen sulfide is converted to elemental sulfur. Moreover, the iron chelate continues in circulation and is later regenerated with oxygen. This enables the iron chelate to be recirculated indefinitely subject to nominal circulation and chemical loss.
While the foregoing is directed to the background of the present disclosure and the specification set forth below describes a specific exemplary procedure, an understanding of this disclosure is aided and assisted by reference to the included drawings.